Apparatus for diagnosing deterioration of nox absorption-reduction catalyst

ABSTRACT

An apparatus for diagnosing deterioration of a NOx absorption-reduction catalyst provided at an exhaust path of an engine includes a sensor disposed upstream of the catalyst to sense a NOx concentration in emission gas, a calculating unit calculating a first ratio of emission of NOx to inflow of NOx or a second ratio of absorption of NOx to inflow of NOx, and a diagnosing unit which diagnoses deterioration of the catalyst using the first or second ratio as an indicator. The calculating unit calculates inflow of NOx based on an output of the sensor and either the flow volume of the emission gas or a correlation value of the flow volume of the emission gas, calculates the absorption of NOx based on the amount of rich components required for reducing the NOx, and calculates the emission of NOx based on the difference between the inflow and absorption of NOx.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based on and claims the benefit of priority fromearlier Japanese Patent Application No. 2009-029220 filed Feb. 11, 2009,the description of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to an apparatus for diagnosingdeterioration of a NOx absorption-reduction catalyst (hereinafterreferred to as an “NOx catalyst”), which apparatus diagnosesdeterioration of a NOx catalyst provided at an exhaust path of aninternal combustion engine (engine).

2. Related Art

In recent years, so-called lean-burn engines or cylinder injectionengines, which control air-fuel ratio to be leaner than a stoichiometricair-fuel ratio (theoretical air-fuel ratio), have been in practical usefor the purposes of improving fuel consumption of vehicles. Theseengines tend to produce more NOx (nitrogen oxides) than normally usedengines. Therefore, some of such engines use a NOx absorption-reductioncatalyst (NOx catalyst) to ensure decrease of the amount of emission ofNOx (hereinafter referred to as “NOx emission”).

The NOx catalyst functions in such a way that it absorbs NOx when theair-fuel ratio of the exhaust gas is lean, and purges (discharges) NOxby reducing the absorbed NOx when the air-fuel ratio has been enriched(or has reached the stoichiometric air-fuel ratio). Therefore, in thecase where lean-burn operation continues for a long time, NOx purgecontrol (also called “rich purge control” or “rich spike control”) isensured to be performed, so that the amount of absorption of NOx(hereinafter referred to as “NOx absorption”) in the NOx catalyst isprevented from reaching saturation levels. With the NOx purge control, atarget air-fuel ratio is intermittently switched to a rich air-fuelratio during the lean-burn operation to purge NOx by reducing the NOxabsorbed in the NOx catalyst.

When the NOx catalyst has been deteriorated and thus the performance ofabsorbing NOx is degraded, the NOx emission in the atmospheric air willincrease. Therefore, deterioration of the NOx catalyst (degradation inthe performance of absorbing NOx) is required to be detected at anearlier occasion.

In this regard, some techniques have been suggested recently, with whichdeterioration of NOx catalyst can be diagnosed.

For example, JP-A-2008-057404 discloses an apparatus for diagnosingdeterioration of catalyst, in which a NOx sensor is disposed downstreamof a NOx catalyst to sense the NOx concentration in the gas emitted(hereinafter referred to as “emission gas”) from the NOx catalyst. Thisdocument further discloses that the output of the NOx sensor is adaptedto be accumulated in a predetermined time period including the period inthe vicinity of completing the NOx purge (rich spike). Then, in thisapparatus, deterioration of the NOx catalyst is diagnosed based onwhether or not the sum of the outputs of the NOx sensor (amount of NOxemission) has exceeded a predetermined deterioration determiningthreshold.

Further, for example, JP-A-2008-064075 discloses an apparatus fordiagnosing deterioration of catalyst, in which a NOx sensor is disposedupstream of a NOx catalyst to accumulate the outputs of the NOx sensor.This document further discloses that the total amount of O₂ and NOx thathave been absorbed in the NOx catalyst (total amount of absorption(hereinafter referred to as “total absorption”)) before start of NOxpurge (rich spike) is calculated while the NOx purge is performed, basedon the output of an O₂ sensor disposed downstream of the NOx catalyst.Then, in this apparatus, deterioration of the NOx catalyst is diagnosedbased on the sum of the outputs (amount of inflow of NOx (hereinafterreferred to as “NOx inflow”) into the NOx catalyst) of the NOx sensorand the total absorption.

Generally, as the size of a NOx catalyst (catalytic capacity) increases,the NOx absorption and the total absorption will increase. Also, as theflow volume of emission gas (hereinafter referred to as “emission gasflow”) that flows into a NOx catalyst increases, the NOx emission fromthe NOx catalyst will increase.

Deterioration diagnosis of a NOx catalyst may be ensured to be conductedbased on the sum of the outputs of a NOx sensor (NOx emission) asdisclosed in JP-A-2008-057404. Also, deterioration diagnosis of a NOxcatalyst may be ensured to be conducted based on the sum of the outputsof a NOx sensor (NOx inflow into the NOx catalyst) and a totalabsorption as disclosed in JP-A-2008-064075. However, with theseconfigurations, the accuracy in the deterioration diagnosis will beimpaired unless the deterioration determining threshold is set accordingto the catalytic capacity or the engine operating condition (emissiongas flow).

However, the configuration of setting the deterioration determiningthreshold according to the catalytic capacity or the engine operatingcondition (emission gas flow) may create a drawback. Specifically, withsuch a configuration, developing and designing the systems fordiagnosing deterioration of NOx catalyst will have to involvetime-consuming processes of checking the deterioration determiningthresholds, leading to low productivity. As a countermeasure againstthis drawback, the number of the processes of checking deteriorationdetermining thresholds may be decreased by limiting the engine operatingcondition, under which deterioration diagnosis is conducted, so that theemission gas flow will fall on a predetermined certain value. However,with this countermeasure, the frequency of conducting deteriorationdiagnosis may fall off and thus the required frequency of conductingdeterioration diagnosis is unlikely to be ensured.

SUMMARY OF THE INVENTION

The present invention has been made in light of the problem set forthabove and has as its object to provide an apparatus for diagnosingdeterioration of a NOx catalyst, which is able to readily enhance theaccuracy in the deterioration diagnosis of a NOx catalyst, readilyenhance productivity (readily decrease the number of checking processes)and readily ensure the frequency of conducting deterioration diagnosis,by mitigating the influences that may be exerted by the size of the NOxcatalyst (catalytic capacity) or by the operational states upon thedeterioration diagnosis of the NOx catalyst.

In order to achieve the object, the present invention provides, as oneaspect, an apparatus for diagnosing deterioration of a NOxabsorption-reduction catalyst provided at an exhaust path of an internalcombustion engine, comprising: a NOx sensor disposed upstream of thecatalyst to sense a NOx concentration in emission gas that flows intothe catalyst; a deterioration diagnostic indicator calculating unitwhich calculates a first ratio of the amount of emission of NOx from thecatalyst, to the amount of inflow of NOx into the catalyst, or a secondratio of the amount of absorption of NOx in the catalyst, to the amountof inflow of NOx into the catalyst; and a deterioration diagnosing unitwhich diagnoses deterioration of the catalyst by using the first ratioor the second ratio as a deterioration diagnostic indicator, wherein thedeterioration diagnostic indicator calculating unit calculates theamount of inflow of NOx into the catalyst based on an output of the NOxsensor and either the flow volume of the emission gas into the catalystor a correlation value of the flow volume of the emission gas,calculates the amount of absorption of NOx in the catalyst based on theamount of rich components required for reducing the NOx absorbed by theNOx catalyst, and calculates the amount of emission of NOx from thecatalyst based on the difference between the amount of inflow of NOxinto the catalyst and the amount of absorption of NOx in the catalyst.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram illustrating an engine control system ingeneral, according to a first embodiment of the present invention;

FIG. 2 is a flow diagram illustrating a process flow of a NOx catalystdeterioration diagnostic routine, according to the first embodiment;

FIG. 3 is a time diagram illustrating fuel injection quantity and outputbehaviors of individual sensors at the time of conducting deteriorationdiagnosis of a NOx catalyst, according to the first embodiment;

FIG. 4 illustrates a relationship between deterioration factor of a NOxcatalyst and non-purification factor of the NOx catalyst;

FIG. 5 is a schematic diagram illustrating an engine control system ingeneral, according to a second embodiment of the present invention;

FIG. 6 is a flow diagram illustrating a process flow of a NOx catalystdeterioration diagnostic routine, according to the second embodiment;

FIG. 7 is a flow diagram illustrating a process flow of a NOx emissionsumming routine, according to the second embodiment;

FIG. 8 is a time diagram illustrating fuel injection quantity and outputbehaviors of individual sensors at the time of conducting deteriorationdiagnosis of a NOx catalyst, according to the second embodiment;

FIG. 9 is a flow diagram illustrating a process flow of a NOx emissionsumming routine, according to a third embodiment of the presentinvention; and

FIG. 10 is a time diagram illustrating fuel injection quantity andoutput behaviors of individual sensors at the time of conductingdeterioration diagnosis of a NOx catalyst, according to the thirdembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to the accompanying drawings, hereinafter will bespecifically described three embodiments of the present inventionapplied to a lean-burn engine.

First Embodiment

Referring to FIGS. 1 to 4, a first embodiment of the present inventionwill be described.

First, referring to FIG. 1, the configuration of an engine controlsystem in general of the first embodiment will be described.

The engine control system includes an engine 11, i.e. an internalcombustion engine. The intake pipe 12 of the engine 11 is provided withan air cleaner 13 disposed most upstream thereof, and an air flow meter14 disposed downstream of the air cleaner 13 to sense the flow volume ofintake air (hereinafter referred to as “intake air flow”). A throttlevalve 15 and a throttle position sensor 16 that detects a throttleposition are disposed downstream of the air flow meter 14.

Further, a surge tank 17 is disposed downstream of the throttle valve15. An intake-pipe pressure sensor 18 is disposed at the surge tank 17to sense the pressure in the intake pipe. The surge tank 17 is providedwith an intake manifold 19 which introduces air to the individualcylinders of the engine 11. Fuel injection valves 20 are attached to theintake manifold 19 so as to be close to the intake ports of therespective cylinders to inject fuel toward the intake ports.

Also, a three-way catalyst 22 and a NOx catalyst 23 (NOxabsorption-reduction catalyst) are disposed in series midway in theexhaust pipe 21 (exhaust path) of the engine 11. The three-way catalyst22 purges HC, CO, and the like, contained in the emission gas. The NOxcatalyst 23 purges NOx contained in the emission gas. The three-waycatalyst 22 disposed upstream of the NOx catalyst 23 is formed to have arelatively small capacity, so that warming up can be completed at anearlier occasion of the startup to decrease emission of the exhaust gasat the startup.

The NOx catalyst 23 on the downstream side absorbs NOx when the air-fuelratio of the emission gas is leaner than a stoichiometric air-fuel ratio(theoretical air-fuel ratio). When the air-fuel ratio has been enriched(or has reached the stoichiometric level), the NOx catalyst 23discharges NOx by reducing and purging the absorbed NOx. The NOxcatalyst 23 is formed to have a relatively large capacity, so that NOxcan be well absorbed in a high-load zone where the amount of NOx in theemission gas becomes large.

The exhaust pipe 21 is also provided with an air-fuel ratio sensor 24(A/F sensor) disposed upstream of the three-way catalyst 22 to sense theair-fuel ratio of the emission gas, a NOx sensor 25 disposed upstream ofthe NOx catalyst 23 (downstream of the three-way catalyst 22) to sensethe NOx concentration in the emission gas that flows into the NOxcatalyst 23, and an O₂ sensor 26 disposed downstream of the NOx catalyst23 to sense the O₂ concentration in the emission gas emitted from theNOx catalyst 23. The output voltage of the O₂ sensor 26 is reverseddepending on whether the air-fuel ratio of the emission gas is richer orleaner than the stoichiometric level. The output of the air-fuel sensor24, on the other hand, substantially linearly changes according to theair-fuel ratio of the emission gas.

An O₂ sensor may be disposed upstream of the three-way catalyst 22,replacing the air-fuel ratio sensor 24. Also, an air-fuel ratio sensormay be disposed downstream of the NOx catalyst 23, replacing the O₂sensor 26.

The NOx sensor 25 upstream of the NOx catalyst 23 is incorporated with afunction of sensing O₂ or a function of detecting an air-fuel ratio inaddition to the function of sensing NOx.

The engine 11 has a cylinder block which is attached with a coolingwater temperature sensor 27 that senses the temperature of the coolingwater, and a crank angle sensor 28 that senses the engine speed.

The outputs of these various sensors are inputted to an engine controlcircuit (hereinafter referred to as an “ECU”) 29. The ECU 29 isprincipally configured by a microcomputer incorporating a ROM (storagemedium) that stores engine control programs. The ignition timing, fuelinjection quantity and the like are controlled according to the engineoperational states by executing these programs.

The NOx catalyst 23 absorbs NOx when the air-fuel ratio of the emissiongas is lean, and purges (discharges) NOx by reducing the absorbed NOxwhen the air-fuel ratio has been enriched (has reached thestoichiometric level). Accordingly, in the case where lean-burnoperation continues for a long time, NOx purge control (also called“rich purge control” or “rich spike control”) is ensured to beperformed, so that the NOx absorption in the NOx catalyst 23 can beprevented from reaching the saturation level. In the NOx purge control,a target air-fuel ratio is intermittently switched to a rich air-fuelratio during the lean-burn operation to purge NOx by reducing the NOxabsorbed in the NOx catalyst 23.

When the NOx catalyst 23 is deteriorated to degrade the function ofabsorbing NOx, the NOx emission into the atmospheric air will beincreased. For this reason, the deterioration of the NOx catalyst 23(degradation in the function of absorbing NOx) is required to bedetected at an earlier occasion.

In this regard, an approach taken in the first embodiment is tocalculate the ratio of the NOx emission from the NOx catalyst 23, to theNOx inflow into the NOx catalyst 23 (hereinafter referred to as“non-purification factor” (first ratio)) and to use the non-purificationfactor as a deterioration diagnostic indicator to thereby diagnosedeterioration of the NOx catalyst 23. The non-purification factor iscalculated from the following Formula (1):

$\begin{matrix}\begin{matrix}{{\text{Non-purification}\mspace{14mu} {factor}} = {{NOx}\mspace{14mu} \text{emission/NOx}\mspace{14mu} {inflow}}} \\{= {\left( {{NOx}\mspace{14mu} \text{inflow-NOx}\mspace{14mu} {absorption}} \right)/}} \\{{{NOx}\mspace{20mu} {inflow}}} \\{= \left\{ {{{NOx}\mspace{14mu} {inflow}} - \left( {{Rich}\mspace{14mu} {component}} \right.} \right.} \\{\left. \left. {\text{inflow-Rich}\mspace{14mu} {component}\mspace{14mu} {emission}} \right) \right\}/} \\{{{NOx}\mspace{14mu} {inflow}}\;}\end{matrix} & (1)\end{matrix}$

(where, NOx emission=NOx inflow−NOx absorption; and NOx absorption=Richcomponent inflow−Rich component emission)

The “NOx inflow” in Formula (1) corresponds to the amount of NOx thatflows into the NOx catalyst 23. The NOx inflow may be calculated bymultiplying the output of the NOx sensor 25 (NOx concentration sensed inthe emission gas that flows into the NOx catalyst 23) disposed upstreamof the NOx 23, by the emission gas flow. Then, the products may besummed to update the sum of the NOx inflow. This process of calculationmay be repeated at a predetermined operation cycle. The emission gasflow may be calculated based on the intake air flow sensed by the airflow meter 14, taking into account the flow delay of the air systempresent from the position of the air flow meter 14 to the position ofthe NOx sensor 25.

The “NOx emission” in Formula (1) corresponds to the amount of NOxemitted from the NOx catalyst 23. The NOx emission may be calculated byobtaining the difference between the NOx inflow into the NOx catalyst 23and the NOx absorption in the NOx catalyst 23.

NOx emission=NOx inflow−NOx absorption  (2)

The NOx absorption in the NOx catalyst 23 may be calculated based on theamount of rich components (hereinafter referred to as “rich componentamount”) required for reducing NOx absorbed by the NOx catalyst 23.Specifically, the NOx absorption in the NOx catalyst 23 may becalculated by obtaining the difference between the amount of richcomponents flowing into the NOx catalyst 23 (hereinafter referred to as“rich component inflow” in the NOx catalyst 23) and the amount of richcomponents emitted from the NOx catalyst 23 (hereinafter referred to as“rich component emission” of the NOx catalyst 23), when reducing the NOxabsorbed in the NOx catalyst 23 by performing NOx purge (rich purge).

NOx absorption=Rich component inflow−Rich component emission  (3)

In the case where the NOx sensor 25 disposed upstream of the NOxcatalyst 23 is incorporated with an air-fuel ratio detecting function,the air-fuel ratio upstream of the NOx catalyst 23 (air-fuel ratio ofthe emission gas that flows into the NOx catalyst 23) may be detected bythe air-fuel ratio detecting function of the NOx sensor 25. Then, therich component inflow into the NOx catalyst 23 may be calculated basedon the detected upstream air-fuel ratio and the emission gas flow(intake airflow), using the following Formula (4):

$\begin{matrix}{{{Rich}\mspace{14mu} {component}\mspace{14mu} {inflow}} = {{{Emission}\mspace{14mu} {gas}{\mspace{11mu} \;}\text{flow/Upstream}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}} - {{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Theoretica}l\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}}}} & (4)\end{matrix}$

In Formula (4), the “Rich component inflow” is calculated by subtractingthe “Emission gas flow/Theoretical air-fuel ratio” from the “Emissiongas flow/Upstream air-fuel ratio” to thereby calculate the richcomponent inflow exceeding the theoretical air-fuel ratio.

In the case where the NOx sensor 25 disposed upstream of the NOxcatalyst 23 is not incorporated with the air-fuel ratio detectingfunction (or in the case where there is no sensor that senses theair-fuel ratio upstream of the NOx catalyst 23), the air-fuel ratioupstream of the NOx catalyst 23 cannot be directly detected. Therefore,the air-fuel ratio detected by the air-fuel ratio sensor 24 upstream ofthe three-way catalyst 22 may be used as the air-fuel ratio upstream ofthe NOx catalyst 23.

The three-way catalyst 22 exerts high emission gas purificationefficiency when the air-fuel ratio of the emission gas fails within thepurification window approximate to the stoichiometric level. This isbecause a target air-fuel ratio of the emission gas is set to be richerthan the purification window of the three-way catalyst 22 while NOxpurge (rich purge) is performed, and thus because the purificationfactor of the three-way catalyst 22 is drastically lowered, resulting inthat the rich components in the emission gas are hardly purified(consumed) in the three-way catalyst 22 but flow into the NOx catalyst23, and further resulting in that the air-fuel ratio upstream of thethree-way catalyst 22 becomes substantially the same as the air-fuelratio upstream of the NOx catalyst 23.

The rich component inflow into the NOx catalyst 23 may be estimatedbased on at least one of the fuel injection quantity, target air-fuelratio, air-fuel ratio correction amount, and the like.

The rich component emission of the NOx catalyst 23 may be calculatedbased on the emission gas flow from the NOx catalyst 23 (intake airflow) and the air-fuel ratio downstream of the NOx catalyst 23 (air-fuelratio of the emission gas from the NOx catalyst 23), using the followingFormula (5):

$\begin{matrix}{{{Rich}\mspace{14mu} {component}\mspace{14mu} {emission}} = {{{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Downstream}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}} - {{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Theoretical}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}}}} & (5)\end{matrix}$

In Formula (5), the “Rich component emission” is obtained by subtractingthe “Emission gas flow/Theoretical air-fuel ratio” from the “Emissiongas flow/Downstream air-fuel ratio” to thereby calculate the richcomponent emission exceeding the theoretical air-fuel ratio.

In the case where an air-fuel sensor is disposed downstream of the NOxcatalyst 23, the air-fuel ratio downstream of the NOx catalyst 23 may bedetected by this air-fuel ratio sensor. In the example of theconfiguration shown in FIG. 1, since the gas sensor downstream of theNOx catalyst 23 is the O₂ sensor 26, the air-fuel ratio downstream ofthe NOx catalyst 23 cannot be directly detected. In this case, theair-fuel ratio downstream of the NOx catalyst 23 may be estimated bymultiplying the O₂ concentration sensed by the O₂ sensor 26 downstreamof the NOx catalyst 23, by an air-fuel ratio conversion coefficient k,using the following Formula (6):

Downstream air-fuel ratio=O₂concentration×k  (6)

Substituting the NOx inflow and the NOx absorption (=Rich componentinflow−Rich component emission) calculated in this way into Formula (1),the non-purification factor of the NOx catalyst 23 can be calculated.Further, using the non-purification factor as a deterioration diagnosticindicator, deterioration diagnosis of the NOx catalyst 23 can beconducted.

The NOx sensor 25 has a property of sensing not only NOx but also anammonia component (NH₃) when the air-fuel ratio of the emission gas isrich. For this reason, during the rich period when the ammonia componentincreases, the NOx concentration sensed by the NOx sensor 25 may resultin a larger value than will be obtained from an actual amount of NOx.Specifically, the NOx concentration will become larger by the degreecorresponding to the concentration of the ammonia component.

Taking this property into consideration in the first embodiment, the NOxconcentration to be sensed is ensured to be set to “0” during the richperiod when the emission gas that flows around the NOx sensor 25 becomesricher than the stoichiometric level, so that the NOx inflow into theNOx catalyst 23 will be inhibited from being summed. According to thisconfiguration, the NOx inflow into the NOx catalyst 23 can be preventedfrom being overestimated due to the presence of the ammonia component.As a result, the accuracy of calculating the NOx inflow into the NOxcatalyst 23 can be prevented from being degraded.

A larger value of the non-purification factor of the NOx catalyst 23calculated from Formula (1) means that the degree of deterioration ofthe NOx catalyst 23 is larger by that much (see FIG. 4). Therefore,deterioration of the NOx catalyst 23 is determined based on whether ornot the non-purification factor is equal to or larger than apredetermined deterioration determining threshold. FIG. 2 shows a NOxcatalyst deterioration diagnostic routine, with which a deteriorationdiagnostic process for the NOx catalyst 23 is performed under thecontrol of the ECU 29 as will be described below.

The NOx catalyst deterioration diagnostic routine shown in FIG. 2 isrepeatedly executed at a predetermined calculation cycle during engineoperation. This routine plays a roll of the deterioration diagnosticindicator calculating means and the deterioration diagnosing means. Uponstart of the present routine, it is determined, in step 101, first,whether or not deterioration diagnosis execution conditions have beenmet. For example, it is determined whether or not the followingconditions (1) to (4) have been met.

(1) That the temperature of the NOx catalyst 23 falls in a predeterminedtemperature range suitable for purging (absorbing/reducing) NOx.

(2) That a predetermined period has expired (or predetermined summedtraveling distance, predetermined summed fuel consumption, etc. has beenreached) from the completion of the previous deterioration diagnosis.

(3) That the engine operational state is a steady operational state.

(4) That no malfunction has been detected in the engine control system,the sensor system or the like by the self-diagnostic function loaded onthe vehicle.

If any one of the conditions (1) to (4) has not been met, thedeterioration diagnosis execution conditions will not be satisfied, andthus the present routine is ended without performing the subsequentprocesses.

On the other hand, if the conditions (1) to (4) have been met, thedeterioration diagnosis execution conditions will be satisfied and theprocess of step 102 and the subsequent processes will be performed toconduct deterioration diagnosis of the NOx catalyst 23 as follows.First, in step 102, the output of the NOx sensor 25 upstream of the NOxcatalyst 23 is obtained to detect the NOx concentration upstream of theNOx catalyst 23 (the NOx concentration in the emission gas that flowsinto the NOx catalyst 23). Then, control proceeds to step 103 where theoutput of the air flow meter 14 (intake air flow Ga) is obtained asinformation (correlation value) correlated to the emission gas flow intothe NOx catalyst 23 to thereby estimate the emission gas flow Ga intothe NOx catalyst 23. In this regard, the emission gas flow Ga may beestimated taking into account the flow delay of the air system presentfrom the position of the air flow meter 14 to the position of the NOxcatalyst 23.

Subsequently, control proceeds to step 104 where it is determinedwhether or not the air-fuel ratio of the emission gas upstream of theNOx catalyst 23 (air-fuel ratio of the emission gas that flows aroundthe NOx sensor 25) is lean, based on the results obtained from the O₂sensing function or the air-fuel ratio detecting function of the NOxsensor 25 upstream of the NOx catalyst 23. As a result, when theair-fuel ratio of the emission gas upstream of the NOx catalyst 23 isdetermined to be rich, it is determined that the NOx concentrationsensed by the NOx sensor 25 may be larger than will be obtained from anactual amount of NOx. Specifically, the NOx concentration may be largerby the degree corresponding to the concentration of the ammoniacomponent. Then, control proceeds to step 105. In step 105, the NOxconcentration to be sensed is set to “0” to inhibit the NOx inflow intothe NOx catalyst 23 from being summed, and then control proceeds to step106.

On the other hand, in step 104, when the air-fuel ratio of the emissiongas upstream of the NOx catalyst 23 is determined to be lean, controlproceeds to step 106 without performing the process in step 105.

Then, in step 106, the NOx concentration ((B) in FIG. 3) upstream of theNOx catalyst 23 sensed by the NOx sensor 25 is multiplied by theemission gas flow Ga and by a calculation time interval dt to calculatethe NOx inflow (=NOx concentration·Ga·dt) into the NOx catalyst 23during the calculation time interval dt this time. Then, the resultantvalue is added to the previously calculated sum of the NOx inflow tothereby update the sum of the NOx inflow.

After that, control proceeds to step 107 where the NOx absorption in theNOx catalyst 23 is summed based on the rich component amount requiredfor completely reducing the NOx absorbed in the NOx catalyst 23.Specifically, the difference is obtained between the rich componentinflow ((A) in FIG. 3) into the NOx catalyst 23 and the rich componentemission ((C) in FIG. 3) from the NOx catalyst 23, when reducing the NOxabsorbed in the NOx catalyst 23 by performing NOx purge (rich purge).The difference is then multiplied by the operation time interval dt tocalculate the NOx absorption in the NOx catalyst 23 during the operationtime interval dt this time. Then, the resultant value is added to thesum of the previously calculated NOx absorption to update the sum of theNOx absorption.

In this regard, the rich component inflow ((A) in FIG. 3) into the NOxcatalyst 23 may be calculated by dividing the emission gas flow Ga intothe NOx catalyst 23 by the air-fuel ratio upstream of the NOx catalyst23, using the following Formula (7):

Rich component inflow=Ga/Upstream air-fuel ratio−Ga/Theoretical air-fuelratio  (7)

The air-fuel ratio detected by the air-fuel ratio sensor 24 upstream ofthe three-way catalyst 22 may be used as the air-fuel ratio upstream ofthe NOx catalyst 23.

The rich component emission ((C) in FIG. 3) from the NOx catalyst 23 maybe calculated by dividing the emission gas flow Ga from the NOx catalyst23 by the air-fuel ratio downstream of the NOx catalyst 23, using thefollowing Formula (8):

Rich component emission=Ga/Downstream air-fuel ratio−Ga/Theoreticalair-fuel ratio  (8)

As shown in the example of configuration of FIG. 1, when the gas sensordownstream of the NOx catalyst 23 is the O₂ sensor 26, the air-fuelratio downstream of the NOx catalyst 23 cannot be directly detected.Therefore, the air-fuel ratio downstream of the NOx catalyst 23 may beestimated by multiplying the O₂ concentration sensed by the O₂ sensor 26downstream of the NOx catalyst 23, by the air-fuel ratio conversioncoefficient k, using the following Formula (9).

Downstream air-fuel ratio=O₂concentration×k  (9)

Subsequently, control proceeds to step 108 where the non-purificationfactor of the NOx catalyst 23 is calculating by substituting the NOxinflow summed in step 106 and the NOx absorption summed in step 107 intothe following Formula (10):

Non-purification factor=(NOx inflow−NOx absorption)/NOx inflow  (10)

Then, control proceeds to step 109 where the non-purification factor ofthe NOx catalyst 23 is compared with the predetermined deteriorationdetermining threshold. When the non-purification factor is equal to orless than the deterioration determining threshold, the NOx catalyst 23is determined, in step 111, not to have been deteriorated (determined tobe normal). When the non-purification factor has exceeded thedeterioration determining threshold, the NOx catalyst 23 is determined,in step 110, to have been deteriorated.

According to the first embodiment described above, deteriorationdiagnosis of the NOx catalyst 23 is conducted using the non-purificationfactor of the NOx catalyst 23 as a deterioration diagnostic indicator,which factor is calculated based on the output of the NOx sensor 25, forexample, disposed upstream of the NOx catalyst 23. Therefore, comparedwith the case where the output sum of the NOx sensor 25 or the totalabsorption is used as a deterioration diagnostic indicator as disclosedin JP-A-2008-057404 or JP-A-2008-064075, the influences can bemitigated, which may be exerted by the size of the NOx catalyst 23(catalytic capacity) or by the engine operational states upon thedeterioration diagnosis of the NOx catalyst. Thus, the accuracy in thedeterioration diagnosis of the NOx catalyst 23 as well as theproductivity (decrease in the number of checking processes) can bereadily enhanced. Also, the frequency of conducting deteriorationdiagnosis can be readily ensured.

In the first embodiment, the non-purification factor has beencalculated, which is a ratio of the NOx emission from the NOx catalyst23 to the NOx inflow into the NOx catalyst 23. Alternative to this, apurification factor (second ratio) may be calculated, which is a ratioof the NOx absorption in the NOx catalyst 23 to the NOx inflow into theNOx catalyst 23. Then the purification factor may be used as adeterioration diagnostic indicator to determine deterioration of the NOxcatalyst 23, based on whether or not the purification factor is equal toor less than the predetermined determining threshold.

The purification factor and the non-purification factor of the NOxcatalyst 23 have a relationship as expressed by the following Formula(11):

Purification factor=1−Non-purification factor  (11)

Substituting the NOx absorption and the NOx inflow calculated in thesame manner as in the first embodiment into the following Formula (12),the purification factor may be calculated.

Purification factor=NOx absorption/NOx inflow  (12)

The purification factor of the NOx catalyst 23 calculated by Formula(12) may be used as a deterioration diagnostic indicator to conductdeterioration diagnosis of the NOx catalyst 23. According to thisdeterioration diagnosis, completely the same effect as in the firstembodiment can be obtained.

Second Embodiment

Referring to FIGS. 5 to 8 hereinafter will be described a secondembodiment of the present invention. In the second and the subsequentembodiments, the components identical with or similar to those in thefirst embodiment are given the same reference numerals for the sake ofomitting explanation.

In the first embodiment, the NOx sensor 25 has been disposed upstream ofthe NOx catalyst 23. However, as shown in FIGS. 5 to 8, in the presentembodiment, a NOx sensor 31 is disposed downstream of the NOx catalyst23, and an O₂ sensor 32 or an air-fuel ratio sensor is disposed upstreamof the NOx catalyst 23 (downstream of the three-way catalyst 22). In thepresent embodiment, the NOx sensor 31 is incorporated with an air-fuelratio detecting function as well as a NOx sensing function. Otherhardware configurations are similar to the first embodiments.

Similar to the first embodiment, in the second embodiment as well, thenon-purification factor is calculated, which is a ratio of the NOxemission from the NOx catalyst 23 to the NOx inflow into the NOxcatalyst 23. Then, using the calculated non-purification factor as adeterioration diagnostic indicator, deterioration diagnosis of the NOxcatalyst 23 is conducted. In the present embodiment, however, since theNOx sensor 31 is positioned downstream of the NOx catalyst 23, the NOxinflow into the NOx catalyst 23 cannot be calculated from the output ofthe NOx sensor 31.

Therefore, in the second embodiment, the non-purification factor of theNOx catalyst 23 is calculated by the following Formula (13):

$\begin{matrix}\begin{matrix}{{\text{Non-purification}\mspace{14mu} {factor}} = {{NOx}\mspace{14mu} \text{emission/NOx}\mspace{14mu} {inflow}}} \\{= {{{NOx}\mspace{14mu} \text{emission/(NOx}\mspace{14mu} {absorption}} +}} \\\left. {{NOx}\mspace{14mu} {emission}} \right) \\{= {{NOx}\mspace{14mu} {{emission}/\left\{ \left( {{Rich}\mspace{14mu} {component}} \right. \right.}}} \\{\left. {\text{inflow-Rich}\mspace{14mu} {component}\mspace{14mu} {emission}} \right) +} \\\left. {{NOx}\mspace{14mu} {emission}} \right\}\end{matrix} & (13)\end{matrix}$

(where, NOx inflow=NOx absorption+NOx emission; and NOx absorption=Richcomponent inflow−Rich component emission)

In Formula (13), the “NOx emission” corresponds to an amount of NOxemitted from the NOx catalyst 23. The NOx emission may be calculated bymultiplying the output of the NOx sensor 31 disposed downstream of theNOx catalyst 23 (the NOx concentration sensed in the emission gasemitted from the NOx catalyst 23), by the emission gas flow. Then, theproducts may be summed to update the sum of the NOx emission. Thisprocess of calculation may be repeated at a predetermined operationcycle. The emission gas flow may be calculated based on the intake airflow sensed by the air flow meter 14, taking into account the flow delayof the air system present from the position of the air flow meter 14 tothe position of the NOx sensor 31.

The “NOx inflow” in Formula (13) corresponds to an amount of NOx thatflows into the NOx catalyst 23, and may be calculated by adding the NOxabsorption in the NOx catalyst 23 to the NOx emission from the NOxcatalyst 23.

NOx inflow=NOx absorption+NOx emission  (14)

The NOx absorption of the NOx catalyst 23 may be calculated based on therich component amount required for reducing the NOx absorbed in the NOxcatalyst 23. Specifically, the NOx absorption may be calculated byobtaining the difference between the rich component inflow into the NOxcatalyst 23 and the rich component emission from the NOx catalyst 23,when reducing the NOx absorbed in the NOx catalyst 23 by performing theNOx purge (rich purge).

NOx absorption=Rich component inflow−Rich component emission  (15)

In the case where the air-fuel ratio sensor is disposed upstream of theNOx catalyst 23 (downstream of the three-way catalyst 22), the air-fuelratio upstream of the NOx catalyst 23 (the air-fuel ratio of theemission gas that flows into the NOx catalyst 23) may be detected by theair-fuel ratio sensor. Then, the rich component inflow into the NOxcatalyst 23 may be calculated based on the detected upstream air-fuelratio and the emission gas flow (intake air flow), using the followingFormula (16).

$\begin{matrix}{{{Rich}\mspace{14mu} {component}\mspace{14mu} {inflow}} = {{{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Upstream}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}} - {{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Theoretical}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}}}} & (16)\end{matrix}$

In the example of the configuration shown in FIG. 5, since the gassensor upstream of the NOx catalyst 23 (downstream of the three-waycatalyst 22) is the O₂ sensor 32, the air-fuel ratio upstream of the NOxcatalyst 23 cannot be directly detected. Therefore, as has beendescribed in the first embodiment, the air-fuel ratio detected by theair-fuel ratio sensor 24 upstream of the three-way catalyst 22 may beused as the air-fuel ratio upstream of the NOx catalyst 23. It should benoted that the rich component inflow into the NOx catalyst 23 may beestimated based on at least one of the fuel injection quantity, targetair-fuel ratio, air-fuel ratio correction amount, and the like.

The rich component emission from the NOx catalyst 23 may be calculatedbased on the emission gas flow (intake air flow) from the NOx catalyst23 and the air-fuel ratio downstream of the NOx catalyst 23 (theair-fuel ratio of the emission gas from the NOx catalyst 23), using thefollowing Formula (17).

$\begin{matrix}{{{Rich}\mspace{14mu} {component}\mspace{14mu} {emission}} = {{{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Downstream}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}} - {{Emission}\mspace{14mu} {gas}\mspace{14mu} \text{flow/Theoretical}\mspace{14mu} \text{air-fuel}\mspace{14mu} {ratio}}}} & (17)\end{matrix}$

In the present embodiment, the NOx sensor 31 downstream of the NOxcatalyst 23 is incorporated with the air-fuel ratio detecting function.Therefore, the air-fuel ratio downstream of the NOx catalyst 23 may bedetected using the air-fuel ratio detecting function of the NOx sensor31. In the case where the NOx sensor 31 downstream of the NOx catalyst23 is incorporated with the O₂ sensing function instead of the air-fuelratio detecting function, the air-fuel ratio may be estimated bymultiplying the O₂ concentration sensed by the O₂ sensing function, bythe air-fuel ratio conversion coefficient k.

The NOx emission and the NOx absorption (=Rich component inflow−Richcomponent emission) calculated as described above may be substitutedinto Formula (13) to calculate the non-purification factor of the NOxcatalyst 23. The obtained non-purification factor may be used as adeterioration diagnostic indicator to conduct deterioration diagnosis ofthe NOx catalyst 23.

Further, in the present embodiment, consideration has been given to thefact that the NOx concentration sensed by the NOx sensor 31 becomeslarger than will be obtained from the actual amount of NOx during therich period when the ammonia component increases. Specifically, the NOxconcentration will become larger by the degree corresponding to theconcentration of the ammonia component. Thus, the NOx concentration tobe sensed is set to “0” in the present embodiment during the rich periodwhen the emission gas that flows into so the NOx catalyst 23 becomesricher than the stoichiometric level. In this way, the NOx emission fromthe NOx catalyst 23 is ensured to be inhibited from being summed.

When the NOx sensor 31 is disposed downstream of the NOx catalyst 23 asin the present embodiment, the concentration of the ammonia componentmay become high even when the emission gas that flows around the NOxsensor 31 is not rich. To explain in detail, when the emission gas thatflows into the NOx catalyst 23 has been enriched, the concentration ofthe ammonia component in the emission gas is estimated to have reached ahigh level. In such a case, the rich components are consumed with thereductive reaction against the absorbed NOx in the course that theemission gas flows through the NOx catalyst 23, while the ammoniacomponent passes through the NOx catalyst 23. For this reason, theconcentration of the ammonia component may become high even when theemission gas that has flowed out of the NOx catalyst 23 and flows aroundthe NOx sensor 31 is not rich.

Considering the above, the NOx emission from the NOx catalyst 23 may beensured to be inhibited from being summed, as in the present embodiment,during the rich period when the emission gas that flows into the NOxcatalyst 23 is enriched. According to this configuration, the NOxemission from the NOx catalyst 23 can be prevented from beingoverestimated due to the presence of the ammonia component. In this way,the accuracy of calculating the NOx emission from the NOx catalyst 23can be prevented from being degraded.

In the present embodiment, summing the NOx emission from the NOxcatalyst 23 is ensured to be inhibited during the period when the outputof the O₂ sensor 32 upstream of the NOx catalyst 23 is rich. Also,summing the NOx emission is ensured to be inhibited even aftercompleting the NOx purge (rich purge) up until the expiration of apredetermined period of time. Inhibition of summing the NOx emission fora while after completing the NOx purge (rich purge) is based on an ideaof considering the flow delay of the enriched emission gas, which delayis caused up until the emission gas reaches the NOx sensor 31 downstreamof the NOx catalyst 23.

The deterioration diagnostic process for the NOx catalyst 23 of thepresent embodiment described above is performed by the ECU 29 accordingto a NOx catalyst deterioration diagnostic routine shown in FIG. 6 aswill be described below.

The NOx catalyst deterioration diagnostic routine shown in FIG. 6 isrepeatedly performed at a predetermined operation cycle during theengine operation. This routine plays the role of the deteriorationdiagnostic indicator calculating means and the deterioration diagnosingmeans. Upon start of the present routine, it is determined, in step 201,first, whether or not deterioration diagnosis execution conditions havebeen met, in the same manner as in the first embodiment. When thedeterioration diagnosis execution conditions have not been met, thepresent routine is ended without performing the subsequent processes.

Conversely, when it is determined, in step 201, that the deteriorationdiagnosis execution conditions have been met, the process in step 202and the subsequent processes are performed to conduct deteriorationdiagnosis of the NOx catalyst 23 as follows. First, in step 202, a NOxemission summing routine shown in FIG. 7, which will be described later,is performed to sum the NOx emission from the NOx catalyst 23.

After that, control proceeds to step 203 where the NOx absorption in theNOx catalyst 23 is summed based the rich component amount required forcompletely reducing the NOx absorbed in the NOx catalyst 23.Specifically, the difference is obtained between the rich componentinflow ((A) in FIG. 8) into the NOx catalyst 23 and the rich componentemission ((C) in FIG. 8) from the NOx catalyst 23, when reducing the NOxabsorbed in the NOx catalyst 23 by performing the NOx purge (richpurge). The difference is multiplied by the calculation time interval dtto calculate the NOx absorption in the NOx catalyst 23 during thecalculation time interval dt this time. The resultant value is thenadded to the previously calculated sum of the NOx absorption to updatethe sum of the NOx absorption.

In this regard, the rich component inflow ((A) in FIG. 8) into the NOxcatalyst 23 may be calculated by dividing the emission gas flow Ga intothe NOx catalyst 23 by the air-fuel ratio upstream of the NOx catalyst23, using the following Formula (18).

Rich component inflow=Ga/Upstream air-fuel ratio−Ga/Theoretical air-fuelratio  (18)

The air-fuel ratio detected by the air-fuel ratio sensor 24 upstream ofthe three-way catalyst 22 may be used as the air-fuel ratio upstream ofthe NOx catalyst 23.

Further, the rich component emission ((C) in FIG. 8) from the NOxcatalyst 23 may be calculated by dividing the emission flow Ga from theNOx catalyst 23 by the air-fuel ratio downstream of the NOx catalyst 23,using the following Formula (19).

Rich component emission=Ga/Downstream air-fuel ratio−Ga/Theoreticalair-fuel ratio  (19)

The air-fuel ratio detected by the air-fuel ratio detecting function ofthe NOx sensor 31 downstream of the NOx catalyst 23 may be used as theair-fuel ratio downstream of the NOx catalyst 23.

Subsequently, control proceeds to step 204 where the non-purificationfactor of the NOx catalyst 23 is calculated by substituting the NOxemission summed in step 202 and the NOx absorption summed in step 203into the following Formula (20):

Non-purification factor=NOx emission/(NOx absorption+NOx emission)  (20)

Then, control proceeds to step 205 where the non-purification factor ofthe NOx catalyst 23 is compared with a predetermined deteriorationdetermining threshold. When the non-purification factor is equal to orless than the deterioration determining threshold, the NOx catalyst 23is determined, in step 207, not to have been deteriorated (to benormal). When the non-purification factor has exceeded the deteriorationdetermining threshold, the NOx catalyst 23 is determined, in step 206,to have been deteriorated.

The NOx emission summing routine shown in FIG. 7 is a sub-routineexecuted in step 202 of the NOx catalyst deterioration diagnosticroutine shown in FIG. 6. Upon start of the NOx emission summing routine,the output of the NOx sensor 31 downstream of the NOx catalyst 23 isobtained, first, in step 301, to detect the NOx concentration downstreamof the NOx catalyst 23 (the NOx concentration of the emission gas fromthe NOx catalyst 23).

After that, control proceeds to step 302 where the output (intake airflow Ga) of the air flow meter 14 is obtained as information correlatedto the emission gas flow from the NOx catalyst 23, so that the emissiongas flow Ga from the NOx catalyst 23 can be estimated. In this regard,the emission gas flow Ga may be estimated taking into account the flowdelay of the air system present from the position of the air flow meter14 to the position of the NOx sensor 31.

Then, control proceeds to step 303 where it is determined whether or notthe current time falls in the period when the output of the O₂ sensor 32upstream of the NOx catalyst 23 is enriched, or the period within apredetermined time from the completion of the NOx purge (rich purge). Asa result, if the current time is determined to fall in the period whenthe output of the O₂ sensor 32 upstream of the NOx catalyst 23 isenriched, or the period within a predetermined time from the completionof the NOx purge (rich purge), control proceeds to step 304. In step304, the NOx concentration sensed by the NOx sensor 31 downstream of theNOx catalyst 23 is set to “0” (the NOx emission from the NOx catalyst 23is inhibited from being summed). Then, control proceeds to step 305.

Conversely, when a “No” determination is made in step 303 (when thecurrent time falls in neither the period when the output of the O₂sensor 32 upstream of the NOx catalyst 23 is enriched, nor the periodwithin a predetermined time from the completion of the NOx purge),control proceeds to the subsequent step 305 without performing theprocess of step 304.

In step 305, the NOx concentration ((B) in FIG. 8) downstream of the NOxcatalyst 23 sensed by the NOx sensor 31 is multiplied by the emissiongas flow Ga and the calculation time interval dt to calculate the NOxemission (=NOx concentration·Ga·dt) from the NOx catalyst 23 during thecalculation time interval dt this time. The resultant value is thenadded to the summed value of the previously calculated NOx emission toupdate the summed value of the NOx emission to thereby end the presentroutine.

In the second embodiment described above as well, the same effects as inthe first embodiment can be obtained.

Third Embodiment

With reference to FIGS. 9 and 10, hereinafter will be described a thirdembodiment of the present invention.

In the second embodiment described above, the NOx concentration to besensed downstream of the NOx catalyst 23 has been set to “0” during theperiod when the output of the O₂ sensor 32 upstream of the NOx catalyst23 is enriched, or the period within a predetermined time from thecompletion of the NOx purge (rich purge). In this way, the NOx emissionfrom the NOx catalyst 23 has been ensured to be inhibited from beingsummed.

The third embodiment shown in FIGS. 9 and 10 makes use of a rich periodwhen the emission gas into the NOx catalyst 23 is richer than thestoichiometric level. Specifically, in this rich period, the output ofthe NOx sensor 31 during this rich period is subjected to an upper limitguard process with the output of the NOx sensor 31 immediately beforethe rich period. Using the value resulting from the upper limit guardprocess, the NOx emission from the NOx catalyst 23 is ensured to becalculated.

The present embodiment has a configuration similar to the secondembodiment. Specifically, the present embodiment also takes into accountthe flow delay of the enriched emission gas, the delay being caused upuntil the enriched emission gas reaches the NOx sensor 31 downstream ofthe NOx catalyst 23. Therefore, the NOx emission from the NOx catalyst23 is calculated using the value resulting from the upper limit guardprocess performed with the output of the NOx sensor 31 immediatelybefore the rich period. This calculation is performed not only duringthe period when the output of the O₂ sensor 32 upstream of the NOxcatalyst 23 is enriched, but also during the period within apredetermined time from the completion of the NOx purge (rich purge).Other configurations are the same as in the second embodiment.

In the present embodiment, a NOx emission summing routine shown in FIG.9 is performed. In this routine, the output of the NOx sensor 31downstream of the NOx catalyst 23 is obtained, first, in step 401, sothat the NOx concentration downstream of the NOx catalyst 23 can bedetected. In the subsequent step 402, the output (intake air flow Ga) ofthe air flow meter 14 is obtained, so that the emission gas flow Ga fromthe NOx catalyst 23 can be estimated.

After that, control proceeds to step 403 where it is determined whetheror not the output from the O₂ sensor 32 upstream of the NOx catalyst 23has just been reversed from lean to rich. When the output has just beenreversed from lean to rich, control proceeds to step 404. In step 404,the NOx concentration sensed by the NOx sensor 31 at the time is storedin the memory, such as a RAM, as an upper limit guard value, and thencontrol proceeds to step 405.

When it is determined, in step 403, that the output from the O₂ sensor32 is not in the state of having just been reversed from lean to rich,control proceeds to step 405 without performing the process, in step404, of storing an upper limit guard value.

In step 405, it is determined whether or not the current time falls inthe period when the output of the O₂ sensor 32 upstream of the NOxcatalyst 23 is enriched, or the period within a predetermined time fromthe completion of the NOx purge (rich purge). As a result, when it isdetermined that the current time fails in the period when the output ofthe O₂ sensor 32 upstream of the NOx catalyst 23 is enriched, or theperiod within a predetermined time from the completion of the NOx purge(rich purge), control proceeds to step 406. In step 406, the upper limitguard value stored in step 404 is used to perform the upper limit guardprocess (detected NOx concentrations upper limit guard value) for thedetected NOx concentration obtained in step 401, and then controlproceeds to step 407.

Conversely, when a “No” determination is made in step 405 (the currenttime falls in neither the period when the output of the O₂ sensor 32upstream of the NOx catalyst 23 is enriched, nor the period within apredetermined time from the completion of the NOx purge), controlproceeds to the subsequent step 407 without performing the upper limitguard process, in step 406, for the detected NOx concentration.

In step 407, the NOx emission from the NOx catalyst 23 is summed in thesame manner as in the second embodiment.

In the third embodiment described above, the NOx emission from the NOxcatalyst 23 can be summed in the period when the output of the O₂ sensor32 upstream of the NOx catalyst 23 is enriched, regarding the output ofthe NOx sensor 31 immediately before the rich period as being the NOxconcentration in the rich period. In this case, the output of the NOxsensor 31 immediately before the rich period corresponds to the NOxconcentration sensed last, which has been less influenced by the ammoniacomponent. Therefore, the NOx emission from the NOx catalyst 23 can beprevented from being overestimated due to the presence of the ammoniacomponent. Thus, the accuracy of calculating the NOx emission from theNOx catalyst 23 can be prevented from being degraded.

In the second and the third embodiments described above, thenon-purification factor has been calculated, which is a ratio of the NOxemission from the NOx catalyst 23 to the NOx inflow into the NOxcatalyst 23. Alternative to this, a purification factor may becalculated, which is a ratio of the NOx absorption in the NOx catalyst23 to the NOx inflow into the NOx catalyst 23. The purification factormay be used as a deterioration diagnostic indicator to determinedeterioration of the NOx catalyst 23 based on whether or not thepurification factor is equal to or less than a predetermineddeterioration determining threshold.

The purification factor of the NOx catalyst 23 also establishes arelationship expressed by Formula (11) provided in the first embodiment.Thus, the purification factor may be calculated by substituting the NOxabsorption and the NOx emission calculated in the same manner as in thesecond and the third embodiments into the following Formula (21):

Purification factor=NOx absorption/(NOx absorption+NOx emission)  (21)

The purification factor of the NOx catalyst 23 calculated by Formula(21) may be used as a deterioration diagnostic indicator to conductdeterioration diagnosis of the NOx catalyst 23. According to thisdeterioration diagnosis, completely the same effects as in the secondand the third embodiments can be obtained.

The application of the present invention is not limited to the lean-burnengines. The present invention may be applied to those engines, such ascylinder-injection engines and dual-injection engines combiningintake-port injection and cylinder injection, in which a NOx catalyst isinstalled. As a matter of course, the present invention may be variouslymodified for application to any engines, irrespective of the presence ofor the type of a catalyst, if any, upstream of the NOx catalyst 23,within the scope not departing from the spirit of the present invention.

Hereinafter, aspects of the above-described embodiments will besummarized.

The above embodiments provide, as one aspect, an apparatus fordiagnosing deterioration of a NOx absorption-reduction catalyst providedat an exhaust path of an internal combustion engine, including: a NOxsensor disposed upstream of the catalyst to sense a NOx concentration inemission gas that flows into the catalyst; a deterioration diagnosticindicator calculating unit which calculates a first ratio(non-purification factor) of the amount of emission of NOx from thecatalyst, to the amount of inflow of NOx into the catalyst, or a secondratio (purification factor) of the amount of absorption of NOx in thecatalyst, to the amount of inflow of NOx into the catalyst; and adeterioration diagnosing unit which diagnoses deterioration of thecatalyst by using the first ratio or the second ratio as a deteriorationdiagnostic indicator, wherein the deterioration diagnostic indicatorcalculating unit calculates the amount of inflow of NOx into thecatalyst based on an output of the NOx sensor and either the flow volumeof the emission gas into the catalyst or a correlation value of the flowvolume of the emission gas, calculates the amount of absorption of NOxin the catalyst based on the amount of rich components required forreducing the NOx absorbed by the NOx catalyst, and calculates the amountof emission of NOx from the catalyst based on the difference between theamount of inflow of NOx into the catalyst and the amount of absorptionof NOx in the catalyst.

According to the embodiment, the non-purification factor or thepurification factor of the NOx catalyst is calculated from the output,for example, of the NOx sensor disposed upstream of the NOx catalyst.The calculated non-purification factor or the purification factor isused as a deterioration diagnostic indicator to conduct deteriorationdiagnosis of the NOx catalyst. Therefore, compared with the case wherethe output sum of the NOx sensor or the total absorption is used as adeterioration diagnostic indicator as disclosed in JP-A-2008-057404 orJP-A-2008-064075, the influences that may be exerted by the size of theNOx catalyst (catalytic capacity) or by the operational states upon thedeterioration diagnosis of the NOx catalyst can be mitigated. Thus, theaccuracy in the deterioration diagnosis of the NOx catalyst as well asthe productivity (decrease in the number of checking processes) can bereadily enhanced. Also, the frequency of conducting deteriorationdiagnosis can be readily ensured.

The NOx sensor has a property of sensing not only NOx but also anammonia component (NH₃) when the air-fuel ratio of the emission gas isrich. For this reason, during the period when the ammonia componentincreases, the NOx concentration sensed by the NOx sensor may result ina larger value than will be obtained from an actual amount of NOx.Specifically, the NOx concentration will become larger by the degreecorresponding to the concentration of the ammonia component.

Considering the property, the deterioration diagnostic indicatorcalculating unit may inhibit the calculation of the amount of inflow ofNOx into the catalyst during a rich period when the emission gas flowingaround the NOx sensor is richer than a theoretical air-fuel ratio(stoichiometric air-fuel ratio). According to this configuration, theNOx inflow into the NOx catalyst can be prevented from beingoverestimated due to the presence of the ammonia component. As a result,the accuracy of calculating the NOx inflow into the NOx catalyst can beprevented from being degraded.

The NOx sensor senses an O₂ concentration or an air-fuel ratio in theemission gas, and the deterioration diagnostic indicator calculatingunit determines whether or not the emission gas flowing around the NOxsensor is richer than the theoretical air-fuel ratio, based on the O₂concentration or the air-fuel ratio sensed by the NOx sensor.

According to this configuration, there is no need of providing a sensorupstream of the NOx catalyst other than the NOx sensor so that O₂concentration or air-fuel ratio can be detected. Thus, thisconfiguration has such advantages as saving space, reducing the numberof parts, and the like. However, the present apparatus may be configuredto have a sensor upstream of the NOx catalyst in addition to the NOxsensor so that O₂ concentration or air-fuel ratio can be detected.

The embodiment described above has used the NOx sensor which is providedupstream of the NOx catalyst. However, the embodiment may be applied toa system in which the NOx sensor is provided downstream of the NOxcatalyst.

In this case, the apparatus may include a NOx sensor disposed downstreamof the catalyst to sense a NOx concentration in emission gas that isemitted from the catalyst; a deterioration diagnostic indicatorcalculating unit which calculates a first ratio of the amount ofemission of NOx from the catalyst, to the amount of inflow of NOx intothe catalyst, or a second ratio of the amount of absorption of NOx inthe catalyst, to the amount of inflow of NOx into the catalyst; and adeterioration diagnosing unit which diagnoses deterioration of thecatalyst by using the first ratio or the second ratio as a deteriorationdiagnostic indicator, wherein the deterioration diagnostic indicatorcalculating unit calculates the amount of emission of NOx from thecatalyst based on an output of the NOx sensor and either the flow volumeof the emission gas from the catalyst or a correlation value of the flowvolume of the emission gas, calculates the amount of absorption of NOxin the catalyst based on the amount of rich components required forreducing the NOx absorbed in the NOx catalyst, and calculates the amountof inflow of NOx into the catalyst by adding the amount of absorption ofNOx in the catalyst to the amount of emission of NOx from the catalyst.

In the embodiment, the non-purification factor or the purificationfactor of the NOx catalyst is calculated from the output, for example,of the NOx sensor disposed downstream of the NOx catalyst. Thecalculated non-purification factor or the purification factor is used asa deterioration diagnostic indicator to conduct deterioration diagnosisof the NOx catalyst. Therefore, the influences that may be exerted bythe size of the NOx catalyst (catalytic capacity) or by the operationalstates upon the deterioration diagnosis of the NOx catalyst can bemitigated. Thus, the accuracy in the deterioration diagnosis of the NOxcatalyst as well as the productivity (decrease in the number of checkingprocesses) can be readily enhanced. Also, the frequency of conductingdeterioration diagnosis can be readily ensured.

In this case as well, consideration is given to the fact that the NOxconcentration sensed by the NOx sensor becomes larger than will beobtained from the actual amount of NOx. Specifically, the NOxconcentration becomes larger by the degree corresponding to the ammoniacomponent concentration during the rich period when the ammoniacomponent increases. Thus, calculation of the NOx emission from the NOxcatalyst is ensured to be inhibited during the rich period when theemission gas that flows into the NOx catalyst becomes richer than thetheoretical air-fuel ratio.

When the NOx sensor is disposed downstream of the NOx catalyst, theconcentration of the ammonia component may become high even when theemission gas that flows around the NOx sensor is not rich. To explain indetail, when the emission gas that flows into the NOx catalyst has beenenriched, the concentration of the ammonia component in the emission gasis estimated to have reached a high level. In such a case, the richcomponents are consumed with the reductive reaction against the absorbedNOx in the course that the emission gas flows through the NOx catalyst,while the ammonia component passes through the NOx catalyst. For thisreason, the concentration of the ammonia component may become high evenwhen the emission gas that has flowed out of the NOx catalyst and flowsaround the NOx sensor is not rich.

Accordingly, the NOx emission from the NOx catalyst may be ensured to beinhibited from being calculated during the period when the emission gasthat flows into the NOx catalyst is enriched. According to thisconfiguration, the NOx emission from the NOx catalyst can be preventedfrom being overestimated due to the presence of the ammonia component.In this way, the accuracy in calculating the NOx emission from the NOxcatalyst can be prevented from being degraded.

The deterioration diagnostic indicator calculating unit may calculate,during a rich period when the emission gas flowing into the catalyst isricher than a theoretical air-fuel ratio, the amount of emission of NOxfrom the catalyst by using a value resulting from an upper limit guardprocess to which an output of the NOx sensor during the rich period issubjected with an output of the NOx sensor immediately before the richperiod.

According to this configuration, the NOx emission from the NOx catalystcan be calculated in the rich period, regarding the output of the NOxsensor immediately before the rich period as being the NOx concentrationin the rich period. In this case, the output of the NOx sensorimmediately before the rich period corresponds to the NOx concentrationsensed last, which has been less influenced by the ammonia component.Therefore, the NOx emission from the NOx catalyst can be prevented frombeing overestimated due to the presence of the ammonia component. Thus,the accuracy of calculating the NOx emission from the NOx catalyst canbe prevented from being degraded.

The apparatus may include an O₂ sensor disposed upstream of the catalystto sense an O₂ concentration in the emission gas, wherein thedeterioration diagnostic indicator calculating unit determines whetheror not the emission gas flowing into the catalyst is richer than thetheoretical air-fuel ratio, based on the O₂ concentration sensed by theO₂ sensor. Alternatively, the apparatus may include an air-fuel ratiosensor disposed upstream of the catalyst to sense an air-fuel ratio inthe emission gas, wherein the deterioration diagnostic indicatorcalculating unit determines whether or not the emission gas flowing intothe catalyst is richer than the theoretical air-fuel ratio, based on theair-fuel ratio sensed by the air-fuel ratio sensor.

Thus, the O₂ sensor or the air-fuel ratio sensor that senses the O₂concentration or air-fuel ratio in the emission gas may be disposedupstream of the NOx catalyst. According to this configuration, anaccurate determination can be made as to whether or not the emission gasthat flows into the NOx catalyst has been enriched.

The apparatus may further include an air-fuel ratio sensor disposedupstream of the catalyst to sense an air-fuel ratio in the emission gas,wherein the deterioration diagnostic indicator calculating unitdetermines whether or not the emission gas flowing into the catalyst isricher than the theoretical air-fuel ratio, based on the air-fuel ratiosensed by the air-fuel ratio sensor.

In short, the difference between the rich component inflow and the richcomponent emission in the NOx catalyst corresponds to the rich componentamount consumed with the reductive reaction against the NOx absorbed inthe NOx catalyst. Therefore, the NOx absorption of the NOx catalyst canbe calculated based on the difference between the rich component inflowand the rich component emission.

It will be appreciated that the present invention is not limited to theconfigurations described above, but any and all modifications,variations or equivalents, which may occur to those who are skilled inthe art, should be considered to fall within the scope of the presentinvention.

1. An apparatus for diagnosing deterioration of a NOxabsorption-reduction catalyst provided at an exhaust path of an internalcombustion engine, comprising: a NOx sensor disposed upstream of thecatalyst to sense a NOx concentration in emission gas that flows intothe catalyst; a deterioration diagnostic indicator calculating unitwhich calculates a first ratio of the amount of emission of NOx from thecatalyst, to the amount of inflow of NOx into the catalyst, or a secondratio of the amount of absorption of NOx in the catalyst, to the amountof inflow of NOx into the catalyst; and a deterioration diagnosing unitwhich diagnoses deterioration of the catalyst by using the first ratioor the second ratio as a deterioration diagnostic indicator, wherein thedeterioration diagnostic indicator calculating unit calculates theamount of inflow of NOx into the catalyst based on an output of the NOxsensor and either the flow volume of the emission gas into the catalystor a correlation value of the flow volume of the emission gas,calculates the amount of absorption of NOx in the catalyst based on theamount of rich components required for reducing the NOx absorbed by theNOx catalyst, and calculates the amount of emission of NOx from thecatalyst based on the difference between the amount of inflow of NOxinto the catalyst and the amount of absorption of NOx in the catalyst.2. The apparatus according to claim 1, wherein the deteriorationdiagnostic indicator calculating unit inhibits the calculation of theamount of inflow of NOx into the catalyst during a rich period when theemission gas flowing around the NOx sensor is richer than a theoreticalair-fuel ratio.
 3. The apparatus according to claim 2, wherein the NOxsensor senses an O₂ concentration or an air-fuel ratio in the emissiongas, and the deterioration diagnostic indicator calculating unitdetermines whether or not the emission gas flowing around the NOx sensoris richer than the theoretical air-fuel ratio, based on the O₂concentration or the air-fuel ratio sensed by the NOx sensor.
 4. Anapparatus for diagnosing deterioration of a NOx absorption-reductioncatalyst provided at an exhaust path of an internal combustion engine,comprising: a NOx sensor disposed downstream of the catalyst to sense aNOx concentration in emission gas that is emitted from the catalyst; adeterioration diagnostic indicator calculating unit which calculates afirst ratio of the amount of emission of NOx from the catalyst, to theamount of inflow of NOx into the catalyst, or a second ratio of theamount of absorption of NOx in the catalyst, to the amount of inflow ofNOx into the catalyst; and a deterioration diagnosing unit whichdiagnoses deterioration of the catalyst by using the first ratio or thesecond ratio as a deterioration diagnostic indicator, wherein thedeterioration diagnostic indicator calculating unit calculates theamount of emission of NOx from the catalyst based on an output of theNOx sensor and either the flow volume of the emission gas from thecatalyst or a correlation value of the flow volume of the emission gas,calculates the amount of absorption of NOx in the catalyst based on theamount of rich components required for reducing the NOx absorbed in theNOx catalyst, and calculates the amount of inflow of NOx into thecatalyst by adding the amount of absorption of NOx in the catalyst tothe amount of emission of NOx from the catalyst.
 5. The apparatusaccording to claim 4, wherein the deterioration diagnostic indicatorcalculating unit inhibits the calculation of the amount of emission ofNOx from the catalyst during a rich period when the emission gas flowinginto the catalyst is richer than a theoretical air-fuel ratio.
 6. Theapparatus according to claim 4, wherein the deterioration diagnosticindicator calculating unit calculates, during a rich period when theemission gas flowing into the catalyst is richer than a theoreticalair-fuel ratio, the amount of emission of NOx from the catalyst by usinga value resulting from an upper limit guard process to which an outputof the NOx sensor during the rich period is subjected with an output ofthe NOx sensor immediately before the rich period.
 7. The apparatusaccording to claim 5, further comprising an O₂ sensor disposed upstreamof the catalyst to sense an O₂ concentration in the emission gas,wherein the deterioration diagnostic indicator calculating unitdetermines whether or not the emission gas flowing into the catalyst isricher than the theoretical air-fuel ratio, based on the O₂concentration sensed by the O₂ sensor.
 8. The apparatus according toclaim 5, further comprising an air-fuel ratio sensor disposed upstreamof the catalyst to sense an air-fuel ratio in the emission gas, whereinthe deterioration diagnostic indicator calculating unit determineswhether or not the emission gas flowing into the catalyst is richer thanthe theoretical air-fuel ratio, based on the air-fuel ratio sensed bythe air-fuel ratio sensor.
 9. The apparatus according to claim 1,wherein the deterioration diagnostic indicator calculating unitcalculates the amount of absorption of NOx in the catalyst based on thedifference between the amount of rich components flowing into thecatalyst and the amount of rich components emitted from the catalyst,when reducing the NOx absorbed in the catalyst.